1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device having high breakdown voltage transistors and a method of fabricating the same.
2. Description of the Related Art
A NAND flash memory which is one of nonvolatile semiconductor memory devices includes selective gate transistors and bit-line contacts adjacent to the selective gate transistors. Each bit-line contact has a barrier film for forming a contact hole or a silicon nitride film serving as a liner film either of which is formed on a spacer insulating film provided on a sidewall of the selective gate transistor. The silicon nitride film is also formed on a source/drain region in which a gate oxide film is formed on the surface of a silicon substrate. See JP-A-2006-60138, for example.
High breakdown voltage transistors are formed in a peripheral circuit region since high voltage needs to be applied to a memory cell transistor at the time of data writing and data erasing in the NAND flash memory. A gate insulating film of each high breakdown voltage transistor has a large film thickness than a gate insulating film of the memory cell transistor or selective gate transistor. See JP-A-H11-31799, for example.
Ion implantation to the source/drain region of the high breakdown voltage transistor is difficult when the thick gate insulating film remains on the silicon substrate in the source/drain region. Accordingly, the ion implantation is considered to be carried out after removal of the gate oxide film on the source/drain region. However, when the aforesaid sidewall insulating film and the silicon nitride film serving as the barrier film are formed in turn subsequent to the ion implantation, the silicon nitride film is formed directly on the surface of the silicon substrate. The direct formation of the silicon nitride film on the substrate surface stresses the silicon substrate, causing crystal fault or trap of the gate oxide film. As a result, the reliability of the high breakdown voltage is reduced.